The present invention relates to digital data communication systems and more particularly to such systems in which pulse coded data is transmitted over a two-wire or four-wire communication link.
It is known that communication links have band limited frequency response characteristics which affect the shape of transmitted pulses. The characteristics of the communication link can adversely affect various characteristics of the transmitted pulse and result in a DC bias being present on the transmission link. This DC bias is detrimental for many reasons and should be avoided.
In order to provide a balanced transmitted waveform, i.e., one which does not contain a variable DC component, it is known to use a biphase transmission code. This type of code contains pulses having a transition for each bit period so that the transmitted signal is balanced. However, even using such a balanced code, a net DC shift results in the communication link due to pulse distortion produced when the signal to be transmitted shifts from one symbol to another.
In addition, it has been found that the first pulse in any series of pulses transmitted across a band limited communication link results in a net DC shift which adversely affects the following pulses. This problem becomes more severe in local networks where the communication link is not used continuously. That is, where the standard operation consists of streams of pulses separated in time, the first pulse of each stream can produce a net DC bias which adversely affects detection of that stream.
Various communication systems have been proposed to reduce the effects of signal distortion caused by the transmission medium. For example, U.S. Pat. No. 4,041,239 to Haass discloses a method and apparatus to reduce the effects of signal distortions in data signals encoded into a combination of wide rectangular pulses and narrow rectangular pulses. A discriminator signal is produced upon the appearance of a wide rectangular pulse and a predistorted signal is produced by multiplying the encoded data signal with the discriminator signal.
U.S. Pat. No. 4,170,715 to Mizokawa discloses a communication system in which nonreturn-to-zero data signals are converted to pulse coded transmission data signals. The nonreturn-to-zero data signals are delayed by a prescribed time delay and an amplitude level changing circuit changes the amplitude level of the digital pulse coded transmission data signals in accordance with the pulse duration of the digital pulse coded transmission data signals on the basis of the delayed nonreturn-to-zero data signals.
Another problem which occurs in local networks is that several communication terminals can be connected or disconnected from a single two-wire communication link. Ideally, the communication link should be impendence matched to the terminals to provide a most efficient energy transfer. However, this requires that each time an additional terminal is connected to the same communication link, the impedences of each of the terminals be adjusted accordingly.